Device-to-Device (D2D) communication refers to direct communication between devices. In D2D communication data to be transmitted from a first device to a second device is typically not relayed via any cellular network. Some examples of D2D communication of the prior art are Bluetooth communication, FlashlinQ communication, WLAN (e.g. IEEE 802.11) communication (e.g. WIFI direct).
Device-to-device communication may be applicable in various scenarios. One scenario is when a cellular radio access network is present, and able to set up a cellular connection between two devices. D2D communication may be a complement to the cellular communication in such scenarios.
There may be situations when D2D communication may provide better performance (better signal quality, higher bit rate, lower latency, etc) than cellular communication. This may be due to proximity between the devices and/or specific signaling gain of the D2D protocol (e.g. hop gain).
In some situations, the network may have constraints (e.g. due to being heavily loaded) resulting in that a service cannot be provided at all using a network connection. Then, D2D communication would be an alternative.
There may also be situations when D2D communication may be preferred by the user of a device (e.g. due to billing costs).
D2D communication may improve spectrum efficiency and reduce the network load for the cellular network, since the D2D connection typically uses another spectrum range (e.g. an unlicensed spectrum) than the cellular network (typically licensed spectrum). Furthermore, since cellular communication uses an uplink-downlink pair for each of the two devices while a D2D connection would only use one link pair, spectrum efficiency is improved even if the D2D connection would use cellular spectrum resources. This would be true even for network assisted D2D communication where most of the data would be transmitted over the D2D connection and only a small amount of information is to be transmitted over the network link.
In D2D communication scenarios, it is envisioned that there may be a large amount of D2D capable devices in vicinity to one another, e.g. in the area covered by a particular network node or in a smaller region. This allows for many possibilities for D2D link establishment, but the situation may also cause quite some interference generated by the D2D signaling, e.g. peer discovery. Optimizing e.g. link performance and system performance in such scenarios is a complex task.
D2D communication may be ad hoc or may be network assisted. For example, a cellular network may assist a D2D connection by establishing security of the D2D link and/or partly or fully controlling the setup of the D2D connection (e.g. device/peer discovery and resource allocation). A cellular network may also assist D2D communication by controlling the interference environment. For example, if using licensed operator's spectrum for the D2D communication, higher reliability can be provided than when operating in unlicensed spectrum. To assist the D2D connection, the network may also provide synchronization and/or partial or full Radio Resource Management (RRM).
Device/peer discovery in D2D communication is typically based on the devices transmitting (e.g. broadcasting) and/or detecting beacon signals respectively. In network assisted D2D device discovery, the network may assist the devices by allocating beacon resources and providing information that the devices can use to construct and detect the beacon signals used for the discovery.
The article “Design aspects of network assisted device-to-device communications” by Fodor, Dahlman, Mildh, Parkvall, Reider, Miklos and Turányi, pages 170-177 in IEEE Communications Magazine, March 2012 discloses that the network can mediate in the discovery process by recognizing D2D candidates and coordinating the time and frequency allocations for sending/scanning for beacons.
U.S. Pat. No. 6,574,266 B1 discloses a base station transmitting a beacon signal including information about the identity and system clock of the base station. The base terminal exchanges information between remote terminals that enables the master terminal to establish a direct communication session with a slave terminal. In order to enable a fast setup of the connection between the terminals, the identity and clock of the master terminal is relayed by the master base station to the other terminal or terminals.
Typically, a beacon signal for a device could be based on the identity of the device, or could be randomly drawn from a set of beacon signals. This applies both if the network allocates the beacons and if the beacons are not provided by the network.
The beacon signals are then transmitted by the respective (master) devices (typically at certain time intervals). The listening (slave) devices then need to scan for beacons. It should be noted that a device may take the role of master only, slave only or a combination of both roles. When a beacon is detected, the corresponding slave typically sends an acknowledgement to the corresponding master, and a D2D connection can be initiated.
Having the beacon signaling allocated and coordinated by a network (as in the IEEE Communications Magazine article cited above) reduces the beacon collision risk. Furthermore, letting the slaves have beacon information of the master(s) (as in both of the disclosures cited above) might improve scanning performance (e.g. shorter time to discovery, lower power consumption).
However, the acknowledgements transmitted in an uncoordinated manner (before the D2D connection is set up) suffers a collision risk and/or may increase the interference level in the system.
Furthermore, allocation of beacons in the network and efficient transmission of the allocation information to the devices becomes cumbersome when the possible number of devices to be considered for device-to-device communication increases.
Therefore, there is a need for alternative approaches to network assisted device-to-device connection setup. There is a need for alternative approaches to beacon resource allocation by a network, as well as transmission of information relating to the allocated beacon resources to devices. There is also a need for alternative approaches to beacon acknowledgement transmission.